The definitive decommissioning of the first generating reactors of the graphite-gas type poses the problem of the storage and custody of slightly active or lightly contaminated waste which is generated in the course of the dismantling operations.
The dismantling of the first generating reactors will in fact produce some 12,000 tons of slightly contaminated metal waste per reactor, 8000 tons of which will come from the heat exchangers.
The contamination of nuclear reactor circuits is generally due to the redeposition of corrosion products carried by the primary fluid and activated in the core, then being redeposited on the inside walls of the circuits. Cobalt 60 is essentially found therein as radionuclide.
This contamination is also caused by fission products escaping through defective sheathing of the fuel elements. This type of contamination is occasional and varies greatly from one reactor to another, and the most characteristic radionuclide is cesium 137.
Metal waste produced by dismantling must usually be subjected to an operation for the decontamination of surfaces bathed by the heat transport fluid, in order to remove the only slightly adherent contamination, known as labile contamination.
The decontamination processes most commonly applied make use of mineral or organic chemical acids, or else foams or gels.
However, these processes generate substantial volumes of liquid effluents, which must be neutralized and treated.
In addition, the metal waste produced by dismantling is transferred, after fragmentation, to special containers for storage at sites for low-level activity waste.
As a rule, it is reckoned that a storage volume of 1 cubic meter is required for the storage of 500 to 800 kg of metal waste, which is considerable.